The present invention relates to an improvement in a method of setting the conditions of a high-frequency (normally, radio frequency (RF)) magnetic field (so-called excitation pulse) applied to an object to be examined so as to excite magnetic resonance in a magnetic resonance imaging (MRI) system, a computer is used to process information on the basis of a specific atomic nucleus spin at a selected slice, i.e., a portion of interest in the object, by using a magnetic resonance phenomenon of the atomic nucleus spin. The processed information is used to reconstruct an image.
A conventional diagnostic MRI system produces a tomographic image of a selected portion of an object to be examined, i.e., a living subject. The object is placed in a uniform static magnetic field. A pulsed gradient field is superposed on the static magnetic field. A steady gradient field may be used in place of the pulsed gradient field. In addition to the static and gradient fields, in order to excite magnetic resonance, a pulsed high-frequency magnetic field must be applied in a direction perpendicular to the static magnetic field. In the MRI system, a magnetic resonance (MR) signal generated by the excited magnetic resonance is detected and processed, and a tomographic image of a specific portion, selected by a combination of the gradient field, and the high-frequency magnetic field is obtained.
The high-frequency magnetic field includes excitation pulses called 90.degree. and 180.degree. pulses. The 90.degree. pulse is a high-frequency component for exciting the magnetic resonance to rotate a magnetic moment of a spin system (i.e., a nucleus spin) through 90.degree. until the magnetic moment is changed from a direction parallel to the static magnetic field to a direction perpendicular thereto. The 180.degree. pulse is a high-frequency component for rotating the magnetic moment of the spin system through 180.degree. in a direction antiparallel to the direction of the static magnetic field.
When 90.degree. and 180.degree. conditions of these high-frequency excitation pulses (i.e., the RF pulses), that is, flip angles determined by the RF pulses, are accurately set, an MR signal having a high S/N (signal-to-noise) ratio can be obtained. Therefore, setting the conditions of the flip angle of the RF pulses is very important.
A Q (quality factor) value of a transmission coil system is changed according to attributes (e.g., adult or child, and fat or slender) of an object, that is, the shape and/or size of the object. When an RF pulse having a predetermined power is applied to different objects, flip angle conditions vary depending on the different objects.
In order to obtain RF pulses for accurately setting a flip angle of 90.degree. or 180.degree. for each examination or photographing cycle in the conventional MRI system, prescanning is performed prior to main scanning in every photographing cycle so that a spin echo signal has a maximum level.
For example, echo signals are acquired by a pulse sequence shown in FIGS. 1A to 1C to set the intensity of a high-frequency magnetic field. More specifically, H.sub.1 in FIG. 1A shows an exciting RF pulse; T.sub.E, a time required until an echo signal is read upon application of exciting RF pulse H.sub.1 ; and T.sub.R, a repetition time of RF pulse H.sub.1.
If longitudinal and transverse relaxation time constants of the spin are defined as T.sub.1 and T.sub.2, respectively, signal intensity S at time t=T.sub.E which satisfies the condition TR&gt;T.sub.1,T.sub.2 is proportional to y'-axis component My' of magnetization as follows: EQU S.varies.My' (1) EQU My'=M.sub.0 ' sin.theta.exp{-(T.sub.E /T.sub.1)} (2)
where M.sub.0 ' is initial magnetization, i.e. the thermal equilibrium magnetization.
The amplitude of RF pulse H.sub.1 is changed and scanning is performed a plurality of times to obtain echo signals. A point (the amplitude of RF pulse H.sub.1) for maximizing the echo signal is selected to obtain amplitude conditions of the 90.degree. RF pulse.
In order to obtain the maximum value of the signal as described above, the input power level is changed to perform scanning a plurality of times and the maximum value of the echo signal is obtained. When an object including a component having long longitudinal relaxation time T.sub.1 is to be examined, the excitation repetition time must be sufficiently prolonged (e.g., TR=3T.sub.1) in order to prevent the influence of T.sub.1. Repetition time T.sub.R is thus normally set to be 2 seconds or longer. Therefore, in order to set the conditions of the 90.degree. and 180.degree. RF pulses, a period of one minute or more is required.
Main scanning time has been shortened in recent years. Therefore, the long prescanning time prior to main scanning is the largest obstacle against reduction in the total scanning time.